This invention relates to the controlled atmospheric storage of fresh fruits and vegetables, and specifically to a container (package) that controls the atmosphere surrounding the packaged fruit or vegetable product by the container having a window in at least one of its walls with a panel therein of a microporous film coated with a thin layer of a cured silicone elastomrer to improve retention of product freshness.
Maintaining the flavor, texture and eating qualities of fresh fruits and vegetables, and extending the shelf life of flowers (hereinafter "produce" collectively) from the time of harvest through the time of consumption is an obvious problem. In addition, there is a large unsatisfied need for preprepared foods, such as cut-up lettuce, carrots, and whole salads that have acceptable shelf life. The most commonly used technique has been refrigeration. Some items, such as tomatoes, bananas and citrus fruits, are routinely picked in a less-than-ripe condition and stored at reduced temperatures until they are sold. Other products, such as grapes and lettuce, are picked at maturity and refrigerated. The reduced temperature helps to retard further ripening, but only for relatively short time periods and may be detrimental to the keeping quality of the product after it is exposed to room temperature.
Other popular techniques used for extending the shelf-life of produce, meats, and poultry, are vacuum packaging and modified atmosphere packaging ("MAP"). MAP involves the injection of an artificial atmosphere into a package and has been used with some success to increase the shelf life of some of these items. Under the MAP system, the stored item receives an ideal atmosphere initially, but the respiration process of the item continuously changes that atmosphere away from the initial state, thus reducing the shelf life.
For each produce type there is an optimum range of concentrations of CO.sub.2 and O.sub.2 at which its respiration is retarded and quality is improved to the greatest extent. For instance, some produce benefit from relatively high levels of CO.sub.2, e.g., strawberries and mushrooms, while others such as lettuce and tomatoes store better at lower levels of CO.sub.2.
Likewise, each produce type also has its own individual respiration rate which can be expressed as cubic centimeters of oxygen per kg/hour.
It is known that the maturation rate of produce can be reduced by controlling the atmosphere surrounding the produce so that an optimum O.sub.2 range and relative concentrations of CO.sub.2 to O.sub.2 are maintained. For instance, Russian Patent 719,555 discloses storage of produce for 6 to 9 months in a temperature range between 0.degree. and 20.degree. C. in a polypropylene bag provided with a ventilation aperture containing a semipermeable membrane that maintains the desired composition of atmosphere inside; the membrane is a plastic material with perforations coated with polyvinyltrimethylsilane with selective gas permeability. French Patent 2,531,042 discloses a container to prevent food dehydration inside a refrigerator where the container has a window with a membrane therein for selectively permitting air to enter while carbon dioxide and ethylene gas escape from the container; the membrane is a sheet of polyamide coated with a layer of polydimethylsiloxane or is a sheet of polyethylene. U.S. Pat. No. 3,507,667 discloses a storage bag of a plastic film (negligible permeability) provided with a window containing therein a panel of poly(organosiloxane) elastomer on a square-mesh fabric having 40 filaments per centimeter of poly (ethylene terephthalate). Japanese Publication No. 61157325 discloses a membrane suitable to produce O.sub.2 -enriched air used for combustion or medical treatment; the membrane is obtained by loading organosiloxane into pores of porous thin films of polyolefins. The published paper "Controlling Atmosphere in a Fresh-Fruit Package" by P. Veeraju and M. Karel, Modern Packaging, Vol. 40, #2 (1966) pages 169-172, 254, discloses using variable-sized panels of polyethylene or permeable parchment paper in the walls of an otherwise impermeable package to establish a controlled atmosphere, and shows experimentally-derived calculations to determine the panel sizes that are appropriate for different respiration rates of produce. However, problems were encountered with the use of film, requiring excessive areas of permeable panels (over 258 cm.sup.2 (40 in.sup.2)), or the use of paper, which is undesirably wettable.
As indicate, the most advanced known controlled atmosphere storage techniques are not entirely satisfactory. There is a need for containers for packaging produce in which the atmosphere can be predictably controlled at approximately the point required to retard the ripening process and retain product freshness, while permitting the use of panels having an area of the order of 25.8 cm.sup.2 (4 in.sup.2) or less, which can easily be so situated that they are not likely to be blocked by other containers in stacking or handling. The area and permeance required are independently and directly dependent on the weight of produce enclosed.